function String_Mul(a, b) {
    var res = Array(a.length + b.length).fill(0);
    for (var i = a.length - 1; i > -1; i--) {
        var add = 0;
        for (var j = b.length - 1; j > -1; j--) {
            var mul = +a[i] * +b[j];
            var sum = res[i + j + 1] + add + mul % 10
            res[i + j + 1] = sum % 10;
            add = Math.floor(mul / 10) + Math.floor(sum / 10);
        }
        res[i] += add;
    }
    return res.join('').replace(/^0*/g, '')
}
